In an onboard navigation system, all data required for the navigation function for the navigation device are preinstalled on an internal or removable storage medium in the motor vehicle. In contrast to this, an online or hybrid navigation system only receives the needed data on demand from a stationary server and stores this data in between locally in the motor vehicle. In doing so, the navigation data regarding the current position and for a corridor along the most probable route, the so-called vehicle horizon, are transmitted from the server to the navigation device. With so-called active route guidance, the data for the corridor along the planned travel route and the vicinity of the driving destination are also downloaded in order to identify alternative routes.
The buffer or cache necessary for buffering the transmitted navigation data is physically limited in any navigation system such that only the absolutely essential navigation data are loaded and cached. Further, optional data, e.g. from a wider corridor along the travel route, are only additionally downloaded, for example, to optimize the navigation functions if there is still free space available in the buffer memory.
The content of a cache or buffer must therefore be administered in such a way that old, unneeded navigation data, the so-called old corridor, be discarded by an algorithm to free up space for new navigation data.
The fundamental problem with an online and hybrid navigation system here is that the navigation data rely on downloading from the server. A mobile connection or another communication link with the corresponding quality is required. However, sufficient necessary quality, i.e. in particular a sufficient transfer rate or transmission bandwidth, is not always provided. It may even be that no online connection is possible if the motor vehicle is in a so-called dead spot. Also, unwanted costs may occur during transfer if it must be, for example, in a so-called roaming zone.
Overall, it is possible that required navigation data requested by a navigation system cannot be downloaded fast enough or not at all due to lack of online availability.
In these cases, there may be functional limitations or even loss of function in the navigation device.
One strategy for overcoming this problem can be to download the navigation data from the server as early as possible to secure its availability. However, this has undesirable high memory requirements for the buffer or cache as a result, which makes the production of such a navigation system in a motor vehicle undesirably expensive. In addition, the navigation data are not current.
A method for predictive downloading of map data to a motor vehicle is known from EP 2120014 A1. Depending on a current driving direction, a future or expected travel route of the motor vehicle is determined and the respective required map material is downloaded along the planned route for individual, rectangular areas. The digital road map is expanded block-by-block in the motor vehicle using this method.
DE 10 2008 018 163 A1 describes a driver assistance system with buffer memory. A driver assistance system is provided in which a navigation unit of a motor vehicle selects a map area from a digital map and transmits it to a driver assistance system. The driver assistance system then calculates the so-called ADAS horizon (ADAS=Advanced Driver Assistance System). From the navigation unit, a map section will be transferred at the time of initialization, which suffices for driving at a defined time T, even if communication fails to the navigation device.
DE 10 2009 008 959 A1 describes a navigation device, in which a digital map can be updated by a dynamic module, for example by integrating traffic jam data or updates on new street guides into the digital map stored in the motor vehicle.